For mixing liquids and solids, mixers in which kinematic cleaning of the surfaces is performed by the mixing elements are often used to avoid deposits on walls and stirring mechanisms. Examples of such devices are twin-screw corotating extruders.
For processes involving relatively high-residence times, mixers with large free volumes are furthermore required. An example of an apparatus which satisfies this requirement is described in European Patent Application EP 0 917 941 A1 (U.S. Pat. No. 6,033,103).
For cleaning the surfaces of such mixers as completely as possible, two degrees of freedom are required. This requirement is met in the case of the apparatus according to EP 0 917 941 A1 (U.S. Pat. No. 6,033,103) by the use of two drive shafts.
In the case of a type of mixer referred to as a Buss-Ko kneader (cf. Mischen beim Herstellen und Verarbeiten von Kunststoffen [Mixing during the production and processing of plastics], published by the Association of German Engineers, VDI-Ges. Kunststofftechnik, Dusseldorf, 1986, page 200), an axially oscillating motion of the kneader shafts is superimposed on a rotational motion.
For high-pressure processes, apparatuses with octagonal housings (see for example EP 0 917 941 A1=U.S. Pat. No. 6,033,103) are unsuitable.
Furthermore, for batch processes, apparatuses with good axial mixing are required.
Consequently, a mixer with a cylindrical housing and which, in the product region, provides good, and in particular axial mixing and which, in particular is, as far as possible, completely self-cleaning, is needed.
Such a mixer is provided by the present invention.